This invention relates generally to tuning systems for receivers, and more particularly to a microprocessor controlled tuning system which has an automatic frequency control feature and which is particularly well suited for use in a satellite signal receiver.
Conventional satellite signal receiving systems typically include an outdoor dish-type antenna arrangement which receives a line of-sight microwave signal from a satellite transponder, and which relays the signal via coaxial cable to a remotely located receiver. The antenna arrangement usually includes a low-noise amplifier, for boosting the level of signals received from the satellite, and a block down frequency converter for converting the frequencies of the received signals to lower levels to reduce the costs and increase the efficiencies associated with transmitting the signals to the receiver.
The block down converter at the antenna has a local oscillator which produces a high frequency (e.g., 5150 Megahertz) signal used to convert the signals coming from the low-noise amplifier to the desired frequency range. However, since the converter is usually located on or near the outdoor dish antenna, the local oscillator in the converter is exposed to environmental conditions (such as temperature extremes) which cause variations in the local oscillator signal frequency, resulting in variations or drifting of the frequencies provided to the receiver. Although tuning systems for receivers are often provided with circuitry for compensating for frequency variations of the local oscillator located in the tuner portion of the receiver, such circuitry is not effective to compensate for variations in frequency of the signal provided to the receiver by the antenna arrangement, such as might be caused by drifting of the local oscillator in the block down converter. These variations can, however, be effectively compensated by incorporating an automatic frequency control or tracking feature in the tuning system of the receiver.
One example of a microprocessor controlled tuning system which incorporates an automatic frequency control feature is shown in U.S. Pat. No. 4,498,191 to Rogers. In this system, a radio frequency (RF) signal is mixed with a signal from a voltage controlled oscillator (VCO) to produce an intermediate frequency (IF) signal which serves as a carrier signal for video and audio information. The tuning system uses a microprocessor controlled phase locked loop to maintain the input voltage to the VCO at a level which results in proper tuning of a selected channel. To compensate for IF signal variations, a portion of the IF amplifier includes an automatic frequency control (AFC) detector which provides a voltage output to one input of an AFC comparator. This voltage is compared to a predetermined threshold or reference voltage which is provided at a second comparator input. The comparator output is connected to the microprocessor, and the comparator produces a logic "0" or a logic "1" output to the microprocessor depending on whether the voltage output from the AFC detector is above or below the threshold. When the microprocessor is operated in a tracking mode, the threshold or reference voltage is periodically and temporarily changed to either of two preset values to generate a "window" which tracks the tuning frequency.
An apparent modification of the Rogers circuit is shown in U.S. Pat. No. 4,542,533 to Parker. In this patent, the output of the AFC detector is similarly connected to one AFC comparator input, and the output of the comparator is connected to the microprocessor. The second comparator input is connected to an increasing voltage from a time constant network controlled by the microprocessor. In the tracking mode, the microprocessor alternately increments a register and reads the output of the comparator until the voltage from the time constant network exceeds the error voltage from the AFC detector. The value in the register is then proportional to the error voltage, and is used to adjust the tuner if necessary.
In circuits of the above-discussed types, the analog AFC detector must be adjusted and calibrated during production. If this is done improperly, or if the reference voltage in the detector subsequently drifts, errors can be introduced into the AFC loop. Furthermore, since the AFC detector produces an analog error voltage signal, use of a circuit (such as the comparator) which will effectively convert the analog signal to a digital signal is reguired for interfacing with the microprocessor.
An object of the present invention is to provide an improved microprocessor controlled tuning system having an automatic frequency control feature which is more cost efficient to manufacture and more flexible in operation than previously known systems.
Another object of the present invention is to provide a microprocessor controlled tuning system in which the parameters of the AFC loop are under software control.
Yet another object of the present invention is to provide a microprocessor controlled tuning system having an AFC feature which can be implemented with relatively few hardware components.
Still another object of the present invention is to provide a microprocessor controlled tuning system having an AFC feature which can be easily adjusted in response to changes in operating conditions, or to facilitate reception of signals from a variety of signal sources.
These and other objects are attained in a microprocessor controlled tuning system which includes a tuner for receiving an RF signal and for supplying an IF signal to a signal detector, a feedback circuit for connecting the IF signal to the microprocessor, and an arrangement for directly measuring the actual frequency of the IF signal and for adjusting the tuner to compensate for variations from the desired frequency caused by drifting of the incoming RF signal. In a preferred embodiment, the tuner includes a mixer which receives signals from an RF signal amplifier and a voltage controlled oscillator to generate the IF signal. The voltage input to the VCO is supplied by a microprocessor controlled phase locked loop. The microprocessor is responsive to a user input channel selection to produce an output associated with the particular selection and to supply the output to a programmable frequency divider. The output of the frequency divider is compared to a reference frequency and a voltage signal is generated to control the VCO. In this preferred embodiment, the IF signal is connected to a counting register in the microprocessor. The microprocessor periodically resets the register to zero and allows the IF signal to increment the register for a predetermined period of time. The resulting value in the register is a direct measure of the actual frequency of the IF signal. If the actual IF signal frequency is above or below a predetermined frequency range, the output supplied to the programmable frequency divider is adjusted accordingly. In the preferred embodiment, the output to the programmable frequency divider is adjusted when the measured frequency of the IF signal remains outside the predetermined frequency range for a predetermined period of time. Adjustment of the output to the programmable frequency divider is prevented if, after adjustment, the output would exceed predetermined upper and lower limits associated with each respective channel.
A preferred manner of operating the tuning system of the present invention includes: reading the user input channel selection; setting the output to the programmable frequency divider to a value associated with the selected channel; taking a direct measure of the actual frequency of the intermediate frequency signal; comparing the measured actual frequency to a predetermined range of frequencies; and adjusting the output to the programmable frequency divider if the actual measured frequency is above or below the predetermined frequency range. In an especially preferred manner of operation, the value of the adjusted output to the programmable frequency divider is compared with a predetermined range of outputs to determine if the adjusted output is within the range. If necessary, the output is readjusted to maintain the output within the range. The direct measure of the actual frequency of the IF signal is made by connecting the IF signal to a counting register on the microprocessor, incrementing the register with the IF signal for a predetermined period of time and reading the contents of the register subsequent to the predetermined time period to obtain a direct measure of the IF signal frequency. A frequency divider may be provided to scale the frequency of the IF signal down to a level which facilitates incrementing the register. However, the digital value in the register is still directly representative of the frequency of the IF signal.
An important advantage of the present invention over previously known techniques is the extent to which software, rather than hardware, is used to control the parameters of the automatic frequency control feature. This greatly increases the flexibility of the tuning system and enhances both operability and applicability of the tuning system to changing conditions in the field. For example, the capture and release ranges of the AFC portion of the system can be adjusted by reprogramming the microprocessor. The system also allows for a channel window to be scanned for the presence of a suspected signal during or following a signal loss. Skewed IF filters, and other irregularities, can be accounted for by programming in correction factors which would be taken into consideration each time a tuning operation and AFC cycle is performed. Channel frequency allocations and channel windows can also easily be changed by reprogramming. This advantage is particularly important when using the receiver to receive satellite signals since some satellites have different numbers of channels and different channel allocations than do others.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.